Ground vehicles such as agricultural tractors and road vehicles are typically equipped with transmissions for transferring power from the engine to the ground. Conventional transmissions provide different gear ratios between the engine and the wheels or tracks of the vehicle to match the various operating conditions. In the past, this has been done with discrete gear ratios. The gear ratios are selected generally manually as for example, in conventional manual transmissions or operator control power shifted transmissions. However, these conventional transmissions have drawbacks in that there are only a limited number of discrete ratios available and the available gear ratios are not always optimal for the particular operating conditions.
Thus, a more flexible system that has been developed is the stepless or infinitely variable transmissions. One particular type of variable transmission is the hydrostatic drive. These transmissions are used commonly in self-propelled combines and lawn tractors. Another type of transmission is an electric drive such as the ones used in railway locomotives and some earth moving equipment. However, both of these transmissions have cost and efficiency disadvantages. Another type of transmission is a hydromechanical transmission. These transmissions have also been used in agricultural tractors, for example, in the tractors sold by Fent of Germany, Klass of Germany and Steyr of Austria. Hydromechanical transmissions have an advantage over hydrostatic drives in that part of the power is transmitted mechanically resulting in better efficiency than pure hydrostatic drives. While the above-mentioned hydromechanical transmissions achieve their intended purpose, there are significant disadvantages. For example, the transmission provided by Fent is a two-range transmission that must be at a stand still to shift between low and high range. The transmission offered by Klass is an eight mode transmission and is rather complex. The Steyr transmission is a four mode version, however, it has a mode shift at a commonly used working range and the efficiency at this point is not optimal.
In vehicles having tracks, there is an additional problem. These track vehicles are steered by providing a differential speed between the two tracks. Track tractors are steered with a clutch and brake for each track. The clutch for one track is released and if necessary, the brake for that track is applied. This slows that track and, thus, the tractor turns. Alternatively, a differential steer is provided using a separate hydraulic pump and motor for the steering. However, the forward steer is still controlled by the transmission with discrete steps. Also, a dual path hydrostatic transmission may be used for track vehicles. Thus, both forward travel and steering are infinitely variable. However, these advantages come at a cost of reduced efficiency.
Therefore, a new and improved hydromechanical transmission is desirable. The new and improved transmission should be infinitely variable.